Investigation into secondary flow in mitred bends

The effect of different cross sections of ducting on the flow in the region of a right angle mitred bend was investigated. Ducts of circular, square, and diamond cross section were used, each of 0.05 sq m in cross sectional area. Observations were made at Reynolds Numbers of 3.83 x 100,000 and 1.85 x 100,000, with wool tufts and spinners. Turbulent regions were observed in the outer corner of each bend and in the upper part of each downstream duct extending to approximately 0.65 m downstream of the bend. This region contained a bubble of reversed flow. The main flow turbulence grew progressively downstream of the bend. A pair of contra-rotating vortices was found originating from the inside of the circular and diamond section bends which were swept downstream. No such pair was present in the rig of square cross section. The pattern of flow did not change with speed but was strongly influenced by the cross sectional shape of the ducting. This confirms a previous suggestion that vortices are produced in a mitred bend if the flow at the bend separates at a swept edge.